1. Field of the Invention
The present invention relates generally to voltage-controlled LC oscillators. In particular, it relates to a method for reducing phase noise in voltage-controlled L oscillators of the type which employ an electronically variable capacitive element, such as a varactor diode or other semiconductor device, as a tuning element.
2. Description of the Prior Art
Phase noise is the noise which results from modulations in the oscillation frequency, or carrier frequency, of an oscillator. Because phase noise affects the ability to precisely tune an oscillator, it represents a very important figure of merit for the oscillators used in communication systems and phase-locked loops.
The phase noise performance of voltage-controlled oscillators (VCOs) has become increasingly important with reduced communications channel spacing and more heavily loaded data transmissions. Presently used varactor tuned VCOs often exhibit excessive phase noise.
VCOs are often tuned electronically using a varactor (sometimes called a varicap diode). Since the capacitance of a varactor varies with reverse bias voltage, this capacitance can be effectively used to electronically control the resonant frequency of an LC voltage-controlled oscillator. Unfortunately, the bias voltage on the varactor in such a VCO circuit does not remain constant during circuit operation. Both the fundamental oscillations of the oscillator circuit and the random electronic noise inherent in the circuit can modulate the value of the varactor's capacitance, thus varying the LC resonant frequency and contributing to the phase noise of the VCO. With respect to circuit oscillations, the phase noise resulting from such an intermodulation process exhibits significant separation from the oscillator carrier frequency and, in principle, can be subsequently removed by an appropriate filter. Moreover, the fundamental frequency-selective nature (the "Q") of any tuned oscillator circuit will cause such noise components far from the carrier frequency to be significantly attenuated.
The manifestations of capacitance modulations induced by the inherent random electronic noise are significantly different. The spectral distribution of the random electronic noise is either white (uniformly distributed over all frequencies) or pink (distributed in proportion to 1/f), depending on the physical mechanism of its origin. In all cases, the random electronic noise invariably exhibits significant, if not dominant, low frequency content. Oscillator phase noise which results from a modulation of the carrier frequency of the oscillator with random electronic noise will, by virtue of the spectral distribution of the random noise and the "Q" of the oscillator circuit, exhibit a broad spectral distribution which peaks at the oscillator carrier frequency. In this case, the near-carrier phase noise, that portion of the oscillator phase noise contained within a few percent of the carrier frequency, proves particularly troublesome since it cannot be subsequently removed by any practically realizable filter.
For a varactor tuned oscillator to continuously tune over a wide range, the varactor must exhibit a large change in capacitance in response to a small change in voltage. However, this enables the varactor's own capacitance to be easily modulated by the small random electronic noise signals generated internally by various circuit elements, including the varactor itself. This, as previously stated, modulates the resonant (or carrier) frequency of the VCO, thus contributing significant phase noise. While this problem has been previously recognized, the prior art fails to teach any technique which mitigates the fundamental trade-off between the continuous tuning range of the VCO and the amount of phase noise generated by the varactor capacitance modulation. That is, the greater the continuous tuning range, the greater the phase noise in the VCO. Rather, the prior art has concentrated largely on methods of finding a reasonable tradeoff between oscillator signal amplitude, tuning range, and other similar design parameters. The article entitled "Fundamental Limitations of Oscillator Performance" by M.J. Underhill contains a detailed discussion of prior art methods for minimizing phase noise in varactor tuned VCOs.
The prior art methods for reducing phase noise in varactor tuned VCOs can be approximately classified into two (not entirely mutually exclusive) groups of approaches: those which employ a compound varactor element, and those which employ the varactor in a fine tuning capacity. The first type of approach involves replacing the single varactor with a compound varactor element comprised of one or more varactors connected in series or parallel with one or more fixed capacitors. The objective is to form a compound varactor element with reduced sensitivity to the RF oscillators in the circuit, usually at the expense of reduced tuning range.
In the second type of approach, a small fine tuning varactor is connected in parallel with a larger capacitor to form the tuning element. Since the fine tuning varactor represents only a fraction of the capacitance of the tuning element, the sensitivity of the total capacitance of the tuning element to voltage variations is thereby reduced. In this approach, the tuning rang can be switched by multiplexing additional capacitive elements in parallel with the fine tuning varactor.
Unfortunately, neither of these methods can be used to prevent internally generated random electronic noise from being modulated into the oscillator without sacrificing the continuous tuning range of the VCO. While the high frequency portion of such noise is partially rejected by virtue of the fact that the oscillator circuit is frequency selective (i.e. has a relatively high "Q"), the low frequency portion induces low frequency fluctuations in the varactor's capacitance which directly result in oscillator phase noise. Thus, there exists a need for a method which prevents low frequency random electronic noise from modulating the carrier frequency and increasing VCO phase noise.
In order to facilitate a broad yet precise description of the present invention, the term "varactor" will hereinafter be used to refer to any circuit in which the capacitance across a pair of terminals varies in response to the voltage across these terminals. A few examples include a single varactor, several varactors in series, several varactors in parallel, a varactor and a fixed capacitor in series, a varactor and a fixed capacitor in parallel, and an FET gate capacitor.